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I think the only real good reason for fearless is when you have multiple fearless unit in the formation. If not they are pretty much useless. They are dead allready they are just to stupid to realise it.
The last surviving unit from an IG formation (i.e. the one with the commisar) gains several benefits from Fearless.
1. He doesn't just die as result of the rest of the formation getting crushed in an assault.
2. When broken he can't be killed off by the simple application of a BM.
3. He can remain closer to enemy formation, potentially getting in the way of where they want to move.
This has knock-on effects.
1. He can rally, and even though its just one unit the enemy must still use a whole activation to remove him.
2. As a commisar he is also inspiring, so he has a chance of doing damage by assaulting (he rallies with no BMs, so you could have +3 from inspiring and BMs). As long as he survives the enemy attacks (which you minimise by clipping) that one unit could break a formation.
3. That one unit rallying could prevent "They Shall Not Pass" from falling to the opponent.
Edit: Perhaps the worst "annoying" Fearless formation is the LatD Plague Zombies. Cheap, teleport(infestation) and numbers to keep them around.